CN114908216B

CN114908216B - Bismuth tellurium adding method for free cutting steel, free cutting carburizing steel and application thereof

Info

Publication number: CN114908216B
Application number: CN202210450665.7A
Authority: CN
Inventors: 陈郧; 皇百红; 张宇; 张峰; 梁蕾蕾; 郭修锋; 王路
Original assignee: Dongfeng Trucks Co ltd
Current assignee: Dongfeng Trucks Co ltd
Priority date: 2022-04-26
Filing date: 2022-04-26
Publication date: 2023-09-01
Anticipated expiration: 2042-04-26
Also published as: CN114908216A

Abstract

The invention discloses a bismuth tellurium adding method of free cutting steel, free cutting carburizing steel and application thereof, and the free cutting carburizing steel provided by the invention comprises the following chemical elements in percentage by weight: c:0.16 to 0.22 percent, si:0.10 to 0.30 percent of Mn:0.55 to 0.75 percent, P:0.015 to 0.030 percent, S:0.045% -0.065%, cr:0.90 to 1.30 percent; te:0.0018 to 0.0065 percent, ti:0.015 to 0.035 percent, N:0.010 to 0.015 percent, bi:0.009% -0.039%, the balance being Fe and unavoidable impurities, te/S weight ratio: 0.04 to 0.10 weight ratio of Bi/S: 0.25 to 0.60. The free-cutting carburizing steel added with Te and Bi has spindle-shaped ratio of sulfide length-width ratio less than or equal to 3 which is more than or equal to 70 percent after rolling and excellent cutting processability. According to the invention, alloy particles of tellurium source and bismuth source are added by utilizing a compressed inert gas injection process, the process is stable, the reaction is gentle, and no severe splashing is caused; and the tellurium and bismuth element yields are high, the actual tellurium yield is more than 85%, and the bismuth yield is more than 60%.

Description

Bismuth tellurium adding method for free cutting steel, free cutting carburizing steel and application thereof

Technical Field

The invention relates to a free-cutting carburizing steel material and application thereof, in particular to a bismuth tellurium adding method of free-cutting steel, the free-cutting carburizing steel and application thereof, and belongs to the field of automobile part materials and processes.

Background

The free cutting steel is a steel grade with excellent cutting machining performance as a prominent feature, and has the advantages of lower cutting force and cutting temperature, good chip disposability, high machining efficiency, good surface finish, prolonged service life of a cutter and the like in the cutting machining process, so that the free cutting steel is widely applied to industries with large cutting machining amount and high machining automation degree, can reduce the cutting machining cost, and greatly improves the product quality and the production efficiency.

Sulfur-based and lead-based free-cutting steels are two types of free-cutting steels that are currently in widespread use. The cutting performance of the lead free-cutting steel is good, but lead is a recognized non-environment-friendly substance, and lead steam generated in the production process has great harm to the health of workers; after lead enters the product, the lead is not easy to remove due to stable chemical property, and the recycling of scrap steel is also a hazard, so that the European Union clearly forbids the addition of lead into steel. Therefore, all the large iron and steel companies in the world are developing toward lead-free cutting steels. The sulfur-series free-cutting steel is the steel with the largest variety and the largest yield in the current free-cutting steels, and accounts for about 70 percent of the total yield of the free-cutting steels. The sulfur and sulfur composite free-cutting steel mainly has the cutting performance, and the sulfur content is generally between 0.20 percent and 0.40 percent and reaches 0.60 percent at most. MnS inclusions in the sulfur free cutting steel cut the continuity of the matrix to make scraps easy to break, and simultaneously play a role in lubrication to reduce the abrasion of the cutter, thereby improving the cutting performance of the steel. However, sulfide inclusions such as MnS in carburizing steel are generally distributed in a long linear segregation manner in the rolling direction, so that the transverse mechanical property of free-cutting steel is obviously reduced, the anisotropy of the steel in the transverse and longitudinal directions is increased, and the fatigue property of shaft-tooth parts is damaged; on the other hand, sulfide distributed in a long linear segregation way has limited effect of improving the cutting performance of the forge piece, and the service life of a cutter in the spline broaching process of the shaft part deep hole drilling and thin-wall annular part is still lower. At present, the S content in the carburizing steel is generally controlled within 0.030 percent, and no patent, literature and related reports for producing free-cutting carburizing steel by adopting Te and Bi composite microalloying technology are seen yet.

In order to improve the machinability of the sulfur free-cutting steel, it is necessary to control the morphology of sulfides in the steel to be spherical or spindle-shaped. In the prior art, the cutting performance is improved mostly by adding elements such as tellurium, bismuth, zirconium, rare earth, titanium and the like. Tellurium and sulfur are the same as main groups, have similar properties, and are important free-cutting elements in free-cutting steel. Tellurium is not only an easy-cutting element, but also has a modifying effect on sulfide, so that manganese sulfide inclusions are converted into spheres or spindle shapes, and a small amount of MnTe formed in steel can also have a lubricating effect in the cutting process, so that the cutting performance of the steel is further improved. However, tellurium has a melting point of 452 ℃ and a boiling point of 1390 ℃, is volatile at a steelmaking temperature of 1500 ℃ or higher, and is expensive, so that how to ensure higher yield and better addition effect is a key for industrial production and application. Bismuth and lead are adjacent elements with the same period, have similar physical and chemical properties, and are one of free-cutting elements. Because the metal bismuth is nontoxic, has a low melting point (271 ℃) and high flexibility similar to that of the metal lead, at present, the metal bismuth is recognized as one of the best environment-friendly free-cutting elements for replacing the lead, and when the bismuth content in the steel is equal to the level of half of the lead content, the same free-cutting effect can be achieved. Therefore, the composite addition of tellurium and bismuth can not only utilize bismuth to obtain the cutting performance similar to that of lead-containing steel, but also utilize tellurium to modify sulfide to further improve the cutting performance of steel, and produce the super free-cutting steel with the cutting performance exceeding that of lead-containing free-cutting steel.

The Chinese patent search and literature query result shows that the Te and Bi composite microalloying is used for preparing the free-cutting steel, and other reports are not yet available except the patent literature CN 109971918A. Patent document CN109971918A discloses a bismuth tellurium composite adding process method of free cutting steel, and the study object is carbon free cutting steel and free cutting stainless steel. And at the end of molten steel refining, preparing the target bismuth-tellurium-containing free-cutting molten steel through a wire feeding and adding process of the bismuth-tellurium-containing cored wire. Although the wire feeding process is stable and does not have severe splashing, the two elements are easy to volatilize at the steelmaking temperature because the boiling point of Te is 1390 ℃ and the boiling point of Bi is 1560 ℃, so that the yield of bismuth and tellurium is lower, the yield of bismuth is less than or equal to 51 percent, the yield of tellurium is less than or equal to 79 percent in the patent document CN109971918A, and the high price and low yield of Te and Bi lead to the high production cost of the free-cutting steel. Therefore, how to increase the yield of Te and Bi in free-cutting steel is a difficult problem to be solved in industrial production.

Disclosure of Invention

Aiming at the technical problems, the invention provides the free-cutting carburizing steel adopting Te and Bi composite microalloying, sulfide forms of shaft tooth parts manufactured by the free-cutting carburizing steel are short bars or spindles, the strip-shaped structure is uniform, and the cold working process performance is excellent.

In a first aspect, the invention provides a free-cutting carburizing steel, which comprises the following chemical components in percentage by weight: 0.16 to 0.22 percent of C, 0.10 to 0.30 percent of Si, 0.55 to 0.75 percent of Mn, 0.015 to 0.030 percent of P, 0.045 to 0.065 percent of S, 0.90 to 1.30 percent of Cr, 0.0018 to 0.0065 percent of Te, 0.015 to 0.035 percent of Ti, 0.010 to 0.015 percent of N, 0.009 to 0.039 percent of Bi, and the balance of Fe and unavoidable impurities, wherein the weight ratio of Te/S: 0.04 to 0.10 weight ratio of Bi/S: 0.25 to 0.60.

In some embodiments of the present invention, the chemical composition is as follows in weight percent: 0.17 to 0.21 percent of C, 0.19 to 0.27 percent of Si, 0.59 to 0.73 percent of Mn, 0.015 to 0.023 percent of P, 0.049 to 0.064 percent of S, 1.05 to 1.20 percent of Cr, 0.0039 to 0.0045 percent of Te, 0.022 to 0.028 percent of Ti, 0.011 to 0.012 percent of N, 0.018 to 0.037 percent of Bi0.018 and the balance of Fe and unavoidable impurities; wherein, te/S weight ratio: 0.06 to 0.09, bi/S weight ratio: 0.33 to 0.58.

In a second aspect, the present invention provides the use of a free-cutting carburized steel as described above for the production of a heavy-duty gearbox shaft tooth part.

In some embodiments of the invention, the invention provides for the use of the free-cutting carburized steel described above in the manufacture of heavy-duty gearbox shaft parts comprising:

forging the free-cutting carburizing steel into a forging stock, and discharging the forging stock from a furnace for quenching after the forging stock is fully austenitized;

carrying out high-temperature tempering on the quenched forging stock;

sequentially carrying out rough machining and finish machining on the forging stock subjected to high-temperature tempering to form a finished semi-finished product;

carburizing and quenching the finished semi-finished product;

and (3) performing low-temperature tempering treatment on the carburized and quenched part.

Preferably, the interval time between the discharging and the high-temperature tempering after austenitizing is not more than 2 hours; the high-temperature tempering temperature is 720-750 ℃, and the high-temperature tempering time is 2.5-3.5 h; the low-temperature tempering temperature is 160-200 ℃, and the low-temperature tempering time is 1-3 h.

In a third aspect, the present invention provides a heavy duty transmission shaft gear component made of the free-cutting carburized steel described above. .

In a fourth aspect, the present invention provides a method for adding bismuth and tellurium in free cutting steel, comprising: and after the adjustment of other components except bismuth and tellurium is finished at the end of molten steel refining, spraying a tellurium source and a bismuth source to the bottom of the molten steel by compressing inert gas to prepare the free-cutting molten steel containing bismuth and tellurium.

In some embodiments of the invention, injecting the tellurium source and the bismuth source into the bottom of the molten steel by compressing an inert gas comprises: one end of the high temperature resistant ceramic tube extends to the bottom of the molten steel, and tellurium-manganese alloy particles and bismuth-manganese-iron alloy particles are sprayed to the bottom of the molten steel through the high temperature resistant ceramic tube by using compressed inert gas.

In some embodiments of the invention, injecting tellurium-manganese alloy particles and bismuth-manganese-iron alloy particles through a refractory ceramic tube to the bottom of the molten steel with a compressed inert gas comprises: firstly, rapidly and uniformly spraying tellurium-manganese alloy particles into molten steel in a VD furnace by using a high-temperature resistant ceramic tube, wherein the pressure of compressed inert gas is 30-50 kPa; after tellurium-manganese alloy particles are added, the temperature is kept for 40-50 s, then bismuth-manganese-iron alloy particles are quickly and uniformly sprayed into molten steel under the action of compressed inert gas by utilizing a high-temperature resistant ceramic tube, and the pressure of the compressed inert gas is 40-60 kPa; after bismuth ferromanganese is added into molten steel, the molten steel is kept for 150 to 200 seconds, so that alloy particles in the molten steel fully react and are melted.

In some embodiments of the invention, the granularity of tellurium-manganese alloy particles is 0.3-1.5 mm, the content of tellurium element is 50% -60%, and the balance is manganese and unavoidable impurities; the granularity of the bismuth-manganese-iron alloy particles is 0.7-2.0 mm, the content of bismuth element is 60-70%, the content of manganese is 20-30%, the content of carbon is less than or equal to 2%, and the balance is Fe and unavoidable impurities.

The invention has the following advantages and beneficial effects:

1. according to the invention, alloy particles of tellurium source and bismuth source are added by utilizing a compressed inert gas injection process, the process is stable, the reaction is gentle, and no severe splashing is caused; and the yield of tellurium and bismuth is high, the yield of actually measured tellurium is more than 85%, the yield of bismuth is more than 60%, the industrial problem of low yield of Te and Bi is solved, the method has high economic value in the production process of the free-cutting steel containing Te and Bi, the production cost is reduced, and the Te and Bi resource consumption is reduced.

2. The invention improves the quality of the free-cutting carburizing steel containing Te and Bi, ensures that sulfides in the forging piece are in a good state of spherical or spindle-shaped dispersion distribution, and ensures that the proportion of sulfide aspect ratio in the prepared free-cutting carburizing steel is less than 3 and can reach more than 70 percent.

3. The prepared Te and Bi composite microalloyed free-cutting carburizing steel is applied to the production of shaft gear parts of a heavy-duty gearbox, and a forging stock adopts a quenching and high-temperature tempering preliminary heat treatment process, compared with the existing production carburizing steel material of the shaft gear parts and an isothermal normalizing process of the forging stock, the metallographic structure of the forging stock of the shaft gear parts is a ferrite and sorbite structure, the structure is fine and uniform, abnormal structures such as bainite or martensite and the like and the phenomenon of out-of-tolerance band structures are avoided, the section hardness can be controlled to be 175-195 HB, and the hardness dispersion of the forging stock in the same batch is less than or equal to 4HB; the chip shape of the Te and Bi composite microalloyed free-cutting carburizing steel carrier part is good, the size and shape are more uniform, and the service life of the cutting tool can be prolonged by more than 30 percent; meanwhile, the roughness and key size parameters of the free-cutting carburized steel shaft tooth part are obviously improved, so that the heat treatment deformation of the part is effectively controlled, the product quality is greatly improved, and the application prospect is very broad.

Drawings

FIG. 1 is a schematic view of the structure of an intermediate shaft part manufactured in accordance with example 1 of the present invention;

FIG. 2 is a schematic structural view of a spindle unit manufactured in accordance with embodiment 2 of the present invention;

fig. 3 is a schematic structural view of a three-gear tooth holder part manufactured in embodiment 3 of the present invention.

Detailed Description

The invention provides free-cutting carburizing steel which comprises the following chemical elements in percentage by weight: c:0.16 to 0.22 percent, si:0.10 to 0.30 percent of Mn:0.55 to 0.75 percent, P:0.015 to 0.030 percent, S:0.045% -0.065%, cr:0.90 to 1.30 percent; te:0.0018 to 0.0065 percent, ti:0.015 to 0.035 percent, N:0.010 to 0.015 percent, bi:0.009% -0.039%, the balance being Fe and unavoidable impurities, te/S weight ratio: 0.04 to 0.10 weight ratio of Bi/S: 0.25 to 0.60. The free-cutting carburizing steel added with Te and Bi has spindle-shaped ratio of sulfide length-width ratio less than or equal to 3 which is more than or equal to 70 percent after rolling and excellent cutting processability.

Further, the free-cutting carburizing steel provided by the invention comprises the following chemical elements in percentage by weight: 0.17 to 0.21 percent of C, 0.19 to 0.27 percent of Si, 0.59 to 0.73 percent of Mn, 0.015 to 0.023 percent of P, 0.049 to 0.064 percent of S, 1.05 to 1.20 percent of Cr, 0.0039 to 0.0045 percent of Te, 0.022 to 0.028 percent of Ti, 0.011 to 0.012 percent of N, 0.018 to 0.037 percent of Bi0.018 and the balance of Fe and unavoidable impurities, wherein the weight ratio of Te/S is as follows: 0.06 to 0.09, bi/S weight ratio: 0.33 to 0.58.

The free-cutting carburizing steel provided by the invention has the effects of main chemical elements:

c: the matrix strength and hardness elements of the material are ensured. When the C content is large, the hardness is high, the cutting performance is poor, and cracks are easy to generate in the round steel rolling process; the content is too low, the hardness of the part is insufficient, and the wear resistance is reduced. Therefore, the C content is controlled within the range of 0.16% to 0.22%.

Si: si and Mn are used as common deoxidizers to influence the deformation of inclusions in steel, and meanwhile, the solid solubility of Si in iron is large, so that ferrite can be remarkably strengthened, and the toughness of a matrix is improved. However, when an excessive amount of Si is added, the hardness of the steel increases, and the silicon oxide constituting the deoxidized product is hard, and thus, the service life of the tool is reduced. Therefore, the Si content is controlled within the range of 0.10% to 0.30%.

Mn: in order to prevent precipitation of low melting point FeS causing hot shortness at grain boundaries, mn is added to precipitate stable MnS, and the chip is easily broken, thereby improving the machinability of steel, and in order to effectively obtain this effect, the Mn control range is 0.55% to 0.75%.

P: the solid solution of P in steel in ferrite can improve hardness and strength, reduce toughness, and lead the chip to be easy to break and remove, thereby obtaining good finish of the machined surface. If the P content is too high, the plasticity is remarkably lowered, the hardness is increased, and the machinability of the steel is adversely affected. Here, the P control range is 0.015% to 0.030%.

S: the sulfide formed by adding S into the steel can destroy the continuity of a metal matrix structure, and is equivalent to a stress concentration source under the action of external force, so that the cutting resistance of the cutter is reduced, and the cutting temperature is reduced; the melting point of sulfide is usually lower, and the sulfide is gradually softened along with the increase of cutting temperature, so that the sulfide has good plastic deformation capacity, plays a role in lubrication, reduces friction force, and reduces friction between chips and a cutter, thereby reducing abrasion of the cutter; in addition, the sulfide has the function of wrapping and antifriction, and when the sulfide with lower hardness is wrapped on the surface of the oxide with high hardness, the abrasion of the cutter is reduced, and meanwhile, the finish of the machined surface is improved. However, too much S forms eutectic compounds (Fe-FeS, fe-FeS-FeO) with oxygen and iron, and cracking is likely to occur during rolling. Thus, the target control range of S is 0.045% -0.065%.

Cr: cr in the steel can obviously improve the strength, the hardness and the wear resistance, but is not beneficial to the improvement of the plasticity and the toughness, and the steel is added with 1.00 to 1.45 percent of Cr, so that the replenishment in the aspect of strength is enhanced on the basis of not affecting the plasticity and the toughness of the steel.

Te: te and S are similar in main group and property, and are important free-cutting elements in free-cutting steel. In addition, tellurium has a modifying effect on sulfide, is easy to be dissolved in MnS inclusions, increases the hardness of plastic inclusions MnS, and prevents the deformation of the inclusions by adhering MnTe to the surfaces of the MnS inclusions, so that the sulfide inclusions are spheroidized. Therefore, the tellurium content is controlled within the range of 0.0018% -0.0065%.

Ti: ti in the steel can be used as an important element for refining grains, and can be used as an auxiliary deoxidizer in Si-Mn co-deoxidization, so that the oxygen level of the deoxidized molten steel is kept in a proper range; meanwhile, tiN particles generated by combining Ti and N can be used as nuclei for precipitation of MnS type sulfides, so that the sulfides are more finely and uniformly dispersed and precipitated. The lubricating and protecting effects of the titanium nitride on the cutter can compensate the influence of the cutting performance brought by the titanium in the steel, and the cutting performance of the bismuth-containing free-cutting steel is improved. However, too high Ti may cause a segregation phenomenon of large-particle titanium nitride or titanium carbonitride, deteriorating the fatigue properties of the steel, and greatly weakening the effect of Ti refinement grains. Therefore, ti is controlled to be 0.015 to 0.035%.

N: the N content is controlled to be 0.010-0.015%, ti and N can be fully combined to form TiN, the formation of TiC or Ti (C, N) is avoided as much as possible, and the formed TiN not only can improve the mechanical property of steel, but also can improve the cutting machining property, and the effect is obvious.

Bi: bi in the steel is dispersed in the form of fine particles, the Bi melting point is low (only 271 ℃), the Bi particles are further softened by frictional heat generated when the bismuth-containing free-cutting steel is cut, and when the bismuth-containing free-cutting steel is cut, the dispersed Bi particles correspond to a cavity existing in the steel, stress is easily concentrated therein, so-called "notch effect" is generated, and the cutting is easily broken therein. Bi plays roles of chipping and cutting, reducing bonding and welding and improving cutting speed in the cutting process of steel, can greatly improve the cutting efficiency, prolongs the service life of a cutter, reduces the roughness of a machined surface and enables the machined surface to be flat and smooth. Therefore, the Bi content is controlled to be 0.009% -0.039%, so that the free-cutting steel of the invention has more excellent cutting processability.

Bi is easy to adhere to manganese sulfide, the manganese sulfide can influence the distribution of Bi, and preferably, the weight ratio of Bi to S is 0.25-0.60, so that the distribution of Bi is facilitated, and the cutting processability of the bismuth-containing free-cutting steel is improved. The Te/S weight ratio in the invention is required to be in accordance with 0.04-0.10. If Te/S is too low, te is mainly dissolved in MnS inclusions and does not form MnTe, more inclusions in steel are II-type MnS inclusions distributed along grain boundaries, the form and distribution of MnS are poor, and a large lifting space is provided for modifying the MnS inclusions by Te. Too high Te/S will form too much MnTe, the MnTe and MnTe-MnS eutectic are in liquid phase at the rolling temperature, too much production will affect the rolling process, and the production cost of Te/S too high will be greatly increased. Therefore, the Te/S weight ratio in the applicable steel grade is required to be in accordance with 0.04-0.10.

The process flow for producing the free-cutting carburizing steel comprises the following steps: converter smelting, LF refining, VD furnace vacuum treatment, inflation pressurization, injection adding tellurium and bismuth, continuous casting and rolling, wherein the detailed production steps are as follows:

1. smelting in a converter: firstly, loading scrap steel into a converter, then directly loading molten iron into the converter, controlling the loading ratio of the molten iron to the scrap steel to be 10 (1-3), carrying out top-bottom combined blowing, carrying out nitrogen supply for 2 minutes before oxygen blowing, then blowing oxygen until the end, and replacing electric energy with chemical energy of molten steel reaction to ensure the decarburization amount of molten steel in the smelting period, and the carbon and temperature at the smelting end point.

2. LF refining: pre-deoxidizing after the eccentric furnace bottom steel-retaining slag-retaining steel-tapping, and adding one or more alloying of ferromanganese, ferrosilicon or compound refining deoxidizing agents into the steel ladle; after the chemical components of elements except S in the steel in the later stage of refining enter the specification, sulfur wires are fed into the refining furnace, the strength of argon blowing at the bottom of the ladle is maintained, and the yield of S is 75-85%.

3. Vacuum treatment of a VD furnace: and (3) placing the refined molten steel in a VD furnace for vacuum degassing treatment, wherein the vacuum degree in the furnace is less than or equal to 133Pa, so as to obtain the molten steel after vacuum degassing, and keeping the oxygen activity of the molten steel to be 15-50 ppm. It is also desirable to ensure that the molten steel has a certain amount of oxygen activity before adding tellurium and bismuth elements to the ladle. Too low oxygen activity is insufficient in fluidity of molten steel, and is disadvantageous in mixing uniformity of adding tellurium source and bismuth source to molten steel in the subsequent step, and in modification efficiency of bismuth and tellurium. Excessive oxygen activity will cause a large amount of oxide inclusions in the molten steel to form, affecting the performance of the steel; in addition, proper oxygen activity can influence sulfide precipitation conditions, is favorable for regulating and controlling sulfide inclusion morphology after adding bismuth and tellurium, improves modification rate, and promotes the transition of sulfide inclusion to spherical or spindle shapes.

4. And (3) inflation pressurization: and (3) filling inert gas nitrogen or argon into the vacuum degassed VD furnace, wherein the pressure in the furnace after filling is 10-18 kPa.

5. Tellurium and bismuth were added by compressed inert gas sparging: in order to improve the yield, the bismuth element content in the bismuth source is diluted, the tellurium source added in the invention is tellurium-manganese alloy particles, the granularity is 0.3-1.5 mm, the tellurium element content is 50-60%, and the balance is manganese and unavoidable impurities; the bismuth source is bismuth ferromanganese alloy particles with the granularity of 0.7-2.0 mm, the content of bismuth element is 60-70%, the content of manganese is 20-30%, the content of carbon is less than or equal to 2%, and the balance is Fe and unavoidable impurities. Through a high temperature resistant ceramic tube with one end extending into molten steel (the distance between the tail end of a ceramic tube orifice and the furnace bottom is 0.3-0.9 m), tellurium-manganese alloy particles are rapidly and uniformly sprayed into the molten steel in a VD furnace under the action of compressed inert gas, and the pressure of the compressed inert gas is 30-50 kPa; after tellurium-manganese alloy particles are added, the bismuth-manganese-iron alloy particles are kept for 40-50 s, and then are quickly and uniformly sprayed into molten steel by utilizing a heat-resistant ceramic tube under the action of compressed inert gas, wherein the pressure of the compressed inert gas is 40-60 kPa. After bismuth ferromanganese is added into molten steel, the molten steel is kept for 150-200 s, so that alloy particles in the molten steel fully react and are melted.

According to the tellurium source and bismuth source adding process, tellurium-manganese alloy particles and bismuth-manganese-iron alloy particles are rapidly and uniformly sprayed into the solution in the vacuum furnace under the action of compressed inert gas, so that boiling and volatilization of tellurium and bismuth can be effectively inhibited. As the melting point and boiling point of bismuth are lower than those of tellurium, in order to prevent volatilization of bismuth steam caused by overlong reaction time of bismuth source, tellurium source and bismuth source alloy particles are added into molten steel for 2 times in sequence, so that the phenomenon that the reaction is too severe due to the fact that tellurium-manganese alloy and bismuth-manganese-iron alloy are added at one time is avoided, the spraying process is ensured not to be severe, and the reaction is stable.

6. Continuous casting: adopting an arc continuous casting machine, and carrying out continuous casting comprising casting, primary cooling, secondary cooling, straightening and flame cutting. Adopting long nozzle argon gas sealing protection and tundish argon blowing protection pouring, adding a low-carbon covering agent into the tundish molten steel surface according to 0.60-0.79 kg/ton steel, wherein the immersion nozzle insertion depth is 100-130 mm, and avoiding the excessive fluctuation of the liquid surface caused by molten steel impact; the secondary cooling part adopts a weak cooling process, and the crystallizer protecting slag adopts high-alkalinity protecting slag with alkalinity of 4; the crystallizer adopts electromagnetic stirring, the current is 600-700A, and the frequency is 4-5 Hz. The casting blank drawing speed is 1.10-1.50 m/min, and the temperature of the continuous casting straightening section is 900-980 ℃.

7. Hot rolling round steel: the initial rolling temperature of the continuous casting blank after austenitizing and heating is 1100-1160 ℃, and the hot rolling temperature is 980-1050 ℃.

The method for manufacturing the free-cutting carburizing steel part comprises the following steps:

(1) Round steel blanking

And (3) blanking the round steel bar stock with the specification of phi 75 mm-phi 120mm into a material section with the specification of 100 mm-475 mm by adopting a numerical control round steel cutting machine, so that the blanking size is accurate, the flatness of the end face of the material section is high, and no burr is generated.

(2) Blank heating and forging forming

And (3) carrying out induction heating on the round steel section after blanking to 1150-1280 ℃, forging and forming into a shaft tooth part forging stock, wherein the final forging temperature is controlled to be 1000-1100 ℃.

(3) Quenching by forging waste heat

And (3) rapidly transferring the final-forged red hot forging stock into a mesh belt furnace for heat preservation by utilizing forging waste heat, wherein the temperature of the mesh belt furnace is set to 900-950 ℃, the running speed of a mesh belt conveyor is 5-10 m/min, and discharging the mesh belt conveyor out of the furnace for quenching, wherein a PAG water-based quenching agent is adopted as a quenching medium.

(4) High temperature tempering

And transferring the quenched forging stock into a tempering furnace in time, wherein the interval time between the quenching and tempering steps is not more than 2 hours, the tempering temperature is controlled to be 720-750 ℃, and the tempering time is 2.5-3.5 hours.

(5) Coarse and fine machining

The forging stock is subjected to rough machining and finish machining to form a finished semi-finished product before heating.

(6) Carburizing and quenching treatment

And (3) loading the finished semi-finished product into a continuous furnace for carburizing treatment, and then selecting direct quenching or press quenching treatment according to the heat treatment deformation of the part.

(7) Low temperature tempering

And (3) placing the carburized and quenched part in a tempering furnace, and performing low-temperature tempering treatment at 160-200 ℃ for 1-3 hours.

The following describes the technical scheme of the present invention in detail through specific embodiments:

example 1

The embodiment relates to free-cutting carburizing steel, a preparation method thereof and application thereof to a middle shaft of a heavy-duty gearbox.

The free-cutting carburizing steel provided by the embodiment comprises the following chemical elements in percentage by weight: 0.21% of C, 0.27% of Si, 0.73% of Mn, 0.023% of P, 0.055% of S, 1.20% of Cr, 0.0045% of Te, 0.026% of Ti, 0.012% of N, 0.018% of Bi0.018% of Fe and unavoidable impurities. Wherein the Te/S weight ratio was 0.08 and the Bi/S weight ratio was 0.33.

The preparation method of the free-cutting carburizing steel provided by the embodiment comprises the following steps:

(1) Smelting in a converter: and (3) sequentially loading high-quality scrap steel and hot charged molten iron into a converter according to the weight ratio of 10:3, carrying out top-bottom combined blowing, carrying out nitrogen supply for 2 minutes before oxygen blowing, and then blowing oxygen until the end, wherein the chemical energy of molten steel reaction is used for replacing electric energy, so that the decarburization amount of molten steel in the smelting period, and the carbon and temperature of a smelting end point are ensured.

(2) LF refining: steel is left at the eccentric furnace bottom, slag is left, steel is tapped, ferromanganese and ferrosilicon are added into a ladle behind the furnace for pre-deoxidation; after the element content except S in the steel in the later stage of refining reaches the target value, a sulfur line is fed into the refining furnace, the strength of ladle bottom argon blowing is maintained, and the yield of S is 80%.

(3) Vacuum treatment of a VD furnace: and (3) placing the refined molten steel in a vacuum furnace for vacuum degassing treatment, wherein the vacuum degree in the furnace is 67Pa, so as to obtain molten steel after vacuum degassing, and keeping the oxygen activity of the molten steel at 40ppm.

(4) And (3) inflation pressurization: argon is filled into the vacuum furnace, and the pressure in the furnace after the argon is filled is 15kPa.

(5) Tellurium and bismuth were added by compressed argon sparging: the tellurium source is tellurium-manganese alloy particles, the granularity is 0.5mm, the tellurium element content is 55%, and the balance is manganese and unavoidable impurities; the bismuth source is bismuth-manganese-iron alloy particles with the granularity of 1.0mm, the content of bismuth element of 65%, the content of manganese of 26%, the content of carbon of 1.5%, and the balance of Fe and unavoidable impurities. Through a high temperature resistant ceramic tube with one end extending into molten steel (the distance between the tail end of a ceramic tube orifice and the bottom of the furnace is 0.5 m), tellurium-manganese alloy particles are rapidly and uniformly sprayed into the molten steel in a VD furnace under the action of compressed inert gas, and the pressure of the compressed inert gas is 40kPa; after tellurium-manganese alloy particles are added, the mixture is kept for 45 seconds, and then bismuth-manganese-iron alloy particles are quickly and uniformly sprayed into molten steel by using a heat-resistant ceramic tube under the action of compressed inert gas, wherein the pressure of the compressed inert gas is 50kPa. After bismuth ferromanganese is added into the molten steel, the molten steel is kept for 180s, so that alloy particles in the molten steel fully react and are melted. According to measurement and calculation, the yield of tellurium element is 90%, and the yield of bismuth element is 71%.

(6) Continuous casting: the continuous casting comprises casting, primary cooling, secondary cooling, straightening and flame cutting. Adopting long nozzle argon gas seal protection and tundish argon blowing protection for casting, adding a low-carbon covering agent into the molten steel surface of the tundish according to 0.65 kg/ton steel, wherein the immersion nozzle insertion depth is 120mm, and avoiding the excessive fluctuation of the liquid surface caused by molten steel impact; the secondary cooling part adopts a weak cooling process, and the crystallizer protecting slag adopts high-alkalinity protecting slag with alkalinity of 4; the crystallizer was stirred electromagnetically with a current of 700A and a frequency of 5Hz. The casting blank drawing speed is 1.20m/min, and the temperature of the continuous casting straightening section is 950 ℃.

(7) Hot rolling round steel: the initial rolling temperature of the continuous casting blank after austenitizing and heating is 1150 ℃ and the hot rolling temperature is 1000 ℃.

The embodiment adopts the processing flow of the heavy gearbox intermediate shaft produced by the free-cutting carburizing steel:

(1) And (3) blanking round steel: and blanking the round steel bar stock with the specification of phi 120mm into a 425mm stock section by adopting a numerical control round steel cutting machine.

(2) Heating and forging the blank: and (3) carrying out induction heating on the obtained round steel material section to 1250 ℃, and carrying out closed die forging forming by adopting an upsetting extrusion method to obtain a middle shaft forging stock, wherein the final forging temperature is 1050 ℃.

(3) Quenching by forging waste heat: and (3) rapidly transferring the final-forged red hot forging stock into a mesh belt furnace for heat preservation, wherein the temperature of the mesh belt furnace is set to 950 ℃, the running speed of a mesh belt conveyor is 5m/min, discharging from the furnace for quenching, and a quenching medium adopts a PAG water-based quenching agent at 60 ℃.

(4) High temperature tempering: and transferring the quenched forging stock into a tempering furnace in time, and tempering at the high temperature of 730 ℃ for 3 hours.

(5) Rough and fine machining: the middle shaft forging stock is subjected to deep hole drilling after rough and finish turning of an outer circle and an end face, and the aperture of a machined through hole is phi 30mm and the length is 563mm; and then, hobbing, internal threads and internal splines are processed on the forge piece subjected to deep hole drilling, and a semi-finished product piece subjected to hot front finish turning is obtained.

(6) Carburizing and quenching treatment: and (3) loading the finished semi-finished intermediate shaft into a continuous carburizing furnace for carburizing treatment, and then directly feeding the part into oil for quenching, wherein the quenching oil temperature is controlled to be 140 ℃.

(7) Low temperature tempering: and (3) placing the intermediate shaft after carburizing and quenching in a tempering furnace, and performing low-temperature tempering treatment at 180 ℃ for 2 hours. The low-temperature tempered intermediate shaft is subjected to straightening, grinding and strong shot blasting treatment to obtain an intermediate shaft finished product, and the part structure of the intermediate shaft finished product is shown in figure 1.

Compared with the existing isothermal normalizing process for producing the carburizing steel material and forging stock of the intermediate shaft, the embodiment has the following beneficial effects:

(1) the Te and Bi composite microalloying free-cutting carburizing steel of the embodiment utilizes high-pressure inert gas injection to add tellurium source and bismuth source, the yield of Te and Bi is high, the ratio of sulfide length-width ratio is less than 3 and can reach more than 70%, and excellent cutting processing performance is ensured while the cost of raw materials is controlled;

(2) the metallographic structure of the intermediate shaft forging stock is a fine and uniform ferrite+sorbite structure, no abnormal structure or strip-shaped structure appears, the section hardness of the forging stock is uniform and is controlled to be 185-189 HB, and the hardness dispersion difference is less than or equal to 4HB;

(3) the service life of the deep hole drilling tool is prolonged from 150 pieces to 270 pieces, and the cutting efficiency is greatly improved.

(4) The roughness of the gear which is processed without gear grinding in the follow-up process of the intermediate shaft finished product piece is obviously improved, the roughness is improved from Ra3.2 to Ra1.6, and the contact fatigue strength of the part is improved.

Example 2

The embodiment relates to free-cutting carburizing steel, a preparation method thereof and application thereof to a main shaft of a heavy-duty gearbox.

The free-cutting carburizing steel provided by the embodiment comprises the following chemical elements in percentage by weight: 0.19% of C, 0.19% of Si, 0.65% of Mn, 0.015% of P, 0.049% of S and 1.05% of Cr; te 0.0043%, ti 0.028%, N0.012%, bi0.021%, the balance Fe and unavoidable impurities, te/S weight ratio is 0.09, bi/S weight ratio is 0.43.

(1) Smelting in a converter: and (3) sequentially loading scrap steel and hot charged molten iron into a converter according to the weight ratio of 10:2, carrying out top-bottom combined blowing, supplying nitrogen for 2 minutes before oxygen blowing, and then blowing oxygen until the end, wherein the chemical energy of molten steel reaction is used for replacing electric energy, so that the decarburization amount of molten steel in the smelting period, the carbon at the smelting end and the temperature are ensured.

(2) LF refining: steel is left at the eccentric furnace bottom, slag is left, steel is tapped, ferromanganese and ferrosilicon are added into a ladle behind the furnace for pre-deoxidation; after the element content except S in the steel in the later stage of refining reaches the target value, a sulfur line is fed into the refining furnace, the strength of ladle bottom argon blowing is maintained, and the yield of S is 85%.

(3) Vacuum treatment of a VD furnace: and (3) placing the refined molten steel in a VD furnace for vacuum degassing treatment, wherein the vacuum degree in the furnace is 50Pa, so as to obtain the molten steel after vacuum degassing, and keeping the oxygen activity of the molten steel to be 35ppm.

(4) And (3) inflation pressurization: argon is filled into the vacuum furnace, and the pressure in the furnace after the argon is filled is 13kPa.

(5) Tellurium and bismuth were added by compressed argon sparging: the tellurium source is tellurium-manganese alloy particles, the granularity is 1.2mm, the tellurium element content is 60%, and the balance is manganese and unavoidable impurities; the bismuth source is bismuth-manganese-iron alloy particles with the granularity of 1.5mm, the content of bismuth element of 70%, the content of manganese of 25%, the content of carbon of 2.0%, and the balance of Fe and unavoidable impurities. Through a high temperature resistant ceramic tube with one end extending into molten steel (the distance between the tail end of a ceramic tube orifice and the bottom of the furnace is 0.8 m), tellurium-manganese alloy particles are rapidly and uniformly sprayed into the molten steel in a VD furnace under the action of compressed inert gas, and the pressure of the compressed inert gas is 50kPa; after tellurium-manganese alloy particles are added, the mixture is kept for 50 seconds, and then bismuth-manganese-iron alloy particles are quickly and uniformly sprayed into molten steel by using a heat-resistant ceramic tube under the action of compressed inert gas, wherein the pressure of the compressed inert gas is 60kPa. After bismuth ferromanganese is added into the molten steel, the molten steel is kept for 200s, so that alloy particles in the molten steel fully react and are melted. According to measurement and calculation, the yield of tellurium is 95%, and the yield of bismuth is 77%.

(6) Continuous casting: the continuous casting comprises casting, primary cooling, secondary cooling, straightening and flame cutting. Adopting long nozzle argon gas sealing protection and tundish argon blowing protection pouring, adding a low-carbon covering agent into the molten steel surface of the tundish according to 0.75 kg/ton of steel, wherein the immersion nozzle insertion depth is 100mm, and avoiding the excessive fluctuation of the liquid surface caused by molten steel impact; the secondary cooling part adopts a weak cooling process, and the crystallizer protecting slag adopts high-alkalinity protecting slag with alkalinity of 4; the crystallizer was stirred electromagnetically with a current of 650A and a frequency of 4Hz. The casting blank drawing speed is 1.10m/min, and the temperature of the continuous casting straightening section is 930 ℃.

(7) Hot rolling round steel: the initial rolling temperature of the continuous casting blank after austenitizing and heating is 1130 ℃, and the hot rolling temperature is 1020 ℃.

The processing flow of the heavy gearbox main shaft produced by adopting the free-cutting carburizing steel is adopted in the embodiment:

(1) And (3) blanking round steel: and blanking the round steel bar stock with the specification of phi 100mm into 475mm stock sections by adopting a numerical control round steel cutting machine.

(2) Heating and forging the blank: and (3) carrying out induction heating on the obtained round steel material section to 1200 ℃, adopting die forging to form a main shaft forging stock, and enabling the final forging temperature to be 1000 ℃.

(3) Quenching by forging waste heat: and (3) rapidly transferring the final-forged red hot forging stock into a mesh belt furnace for heat preservation, wherein the temperature of the mesh belt furnace is set to 930 ℃, the running speed of a mesh belt conveyor is 7m/min, discharging from the furnace for quenching, and a quenching medium adopts a PAG water-based quenching agent at 65 ℃.

(4) High temperature tempering: and transferring the quenched main shaft forging stock into a tempering furnace in time, and tempering at the high temperature of 730 ℃ for 3 hours.

(5) Rough and fine machining: the spindle forging stock is subjected to deep hole drilling after rough and finish turning of an outer circle and an end face, and the aperture of a machined through hole is phi 25mm and the length is 569mm; and then, machining external splines and internal threads on the spindle forging subjected to deep hole drilling to obtain a semi-finished product of finish turning before heating.

(6) Carburizing and quenching treatment: and (3) loading the finished semi-finished product of the main shaft into a continuous carburizing furnace for carburizing treatment, and then directly feeding oil into the furnace for quenching at 860 ℃, wherein the quenching oil temperature is controlled to be 130 ℃.

(7) Low temperature tempering: and (3) placing the carburized and quenched main shaft in a tempering furnace, and performing low-temperature tempering treatment at 200 ℃ for 1.5 hours. The low-temperature tempered main shaft is subjected to straightening, grinding and strong shot blasting treatment to obtain a main shaft finished product, and the part structure of the main shaft finished product is shown in figure 2.

Compared with the existing isothermal normalizing process for producing carburizing steel materials and forging stocks of the main shaft, the embodiment has the following beneficial effects:

(1) the composite microalloying free-cutting carburizing steel prepared by adding Te and Bi by adopting a high-pressure inert gas injection process has high yield of Te and Bi, the ratio of sulfide aspect ratio of less than 3 can reach more than 70 percent, the cost of raw materials is low, and the cutting processability is good;

(2) the metallographic structure of the main shaft forging stock is a ferrite and sorbite structure, the structure is fine and uniform, no out-of-tolerance phenomenon of a banded structure occurs, the section hardness of the forging stock is controlled to be 180-183 HB, and the hardness dispersion difference is less than or equal to 3HB;

(3) the service life of a tool for deep hole drilling of an inner hole of a main shaft forging stock is prolonged from 180 pieces to 300 pieces, and the service life of a tool for hobbing an external spline of the main shaft is prolonged from 190 pieces to 250 pieces, so that the production cost is greatly reduced, and the production efficiency is improved.

(4) The M value reject ratio of the spline position of the finished spindle part is reduced from 21.8% to within 3%, and the product quality is improved practically.

Example 3

The embodiment relates to free-cutting carburizing steel, a preparation method thereof and application of the free-cutting carburizing steel to a three-gear tooth holder of a heavy-duty gearbox.

The free-cutting carburizing steel provided by the embodiment comprises the following chemical elements in percentage by weight: 0.17% of C, 0.26% of Si, 0.59% of Mn, 0.019% of P, 0.064% of S and 1.12% of Cr; te 0.0039%, ti 0.022%, N0.011%, bi 0.037%, the balance of Fe and unavoidable impurities, wherein the weight ratio of Te to S is 0.06, and the weight ratio of Bi to S is 0.58.

(1) Smelting in a converter: the high-quality scrap steel and hot charged molten iron are sequentially filled into a converter according to the weight ratio of 10:1.5, top-bottom combined blowing is carried out, nitrogen supply is carried out for 2 minutes before oxygen blowing, then oxygen blowing is carried out until the end, and the chemical energy of molten steel reaction is used for replacing electric energy, so that the decarburization amount of molten steel in the smelting period, and the carbon and the temperature of smelting endpoint are ensured.

(2) LF refining: the eccentric furnace bottom is left with steel, slag and steel are left, and a composite deoxidizer is added into a ladle to pre-deoxidize after the furnace; after the element content except S in the steel in the later stage of refining reaches a set value, a sulfur line is fed into the refining furnace, the strength of ladle bottom argon blowing is maintained, and the yield of S is 79%.

(3) Vacuum treatment of a VD furnace: and (3) placing the refined molten steel in a vacuum furnace for vacuum degassing treatment, wherein the vacuum degree in the furnace is 40Pa, so as to obtain molten steel after vacuum degassing, and keeping the oxygen activity of the molten steel to be 25ppm.

(4) And (3) inflation pressurization: argon is filled into the vacuum furnace, and the pressure in the furnace after the argon is filled is 18kPa.

(5) Tellurium and bismuth were added by compressed argon sparging: the tellurium source is tellurium-manganese alloy particles, the granularity is 0.3mm, the tellurium element content is 50%, and the balance is manganese and unavoidable impurities; the bismuth source is bismuth ferromanganese alloy particles with the granularity of 0.7mm, the content of bismuth element of 60 percent, the content of manganese of 30 percent, the content of carbon of 0.5 percent, and the balance of Fe and unavoidable impurities. Through a high temperature resistant ceramic tube with one end extending into molten steel (the distance between the tail end of a ceramic tube orifice and the bottom of the furnace is 0.3 m), tellurium-manganese alloy particles are rapidly and uniformly sprayed into the molten steel in a VD furnace under the action of compressed inert gas, and the pressure of the compressed inert gas is 35kPa; after tellurium-manganese alloy particles are added, the mixture is kept for 40 seconds, and then bismuth-manganese-iron alloy particles are quickly and uniformly sprayed into molten steel by using a heat-resistant ceramic tube under the action of compressed inert gas, wherein the pressure of the compressed inert gas is 45kPa. After bismuth ferromanganese is added into the molten steel, the molten steel is kept for 150s, so that alloy particles in the molten steel fully react and are melted. According to measurement and calculation, the yield of tellurium element is 88%, and the yield of bismuth element is 65%.

(6) Continuous casting: the continuous casting comprises casting, primary cooling, secondary cooling, straightening and flame cutting. Adopting long nozzle argon gas seal protection and tundish argon blowing protection for casting, adding a low-carbon covering agent into the molten steel surface of the tundish according to 0.60 kg/ton steel, wherein the immersion nozzle insertion depth is 125mm, and avoiding the excessive fluctuation of the liquid surface caused by molten steel impact; the secondary cooling part adopts a weak cooling process, and the crystallizer protecting slag adopts high-alkalinity protecting slag with alkalinity of 4; the crystallizer was stirred electromagnetically with a current of 700A and a frequency of 5Hz. The casting blank drawing speed is 1.50m/min, and the temperature of the continuous casting straightening section is 980 ℃.

(7) Hot rolling round steel: the initial rolling temperature of the continuous casting blank after austenitizing and heating is 1150 ℃ and the hot rolling temperature is 1040 ℃.

The processing flow of the heavy-duty gearbox one-third gear tooth holder produced by adopting the free-cutting carburizing steel is adopted in the embodiment:

(1) And (3) blanking round steel: and blanking the round steel bar stock with the specification of phi 75mm into a 117mm stock section by adopting a numerical control round steel cutting machine.

(2) Heating and forging the blank: and (3) carrying out induction heating on the obtained round steel material section to 1280 ℃, and carrying out grinding and expanding to form a three-gear tooth seat forging stock after punching, wherein the final forging temperature is 1020 ℃.

(3) Quenching by forging waste heat: and (3) rapidly transferring the final-forged red hot forging stock into a mesh belt furnace for heat preservation, wherein the temperature of the mesh belt furnace is set to 910 ℃, the running speed of a mesh belt conveyor is 10m/min, discharging from the furnace for quenching, and a quenching medium adopts PAG water-based quenching agent at 55 ℃.

(4) High temperature tempering: and (3) transferring the quenched forging stock of the first three gears of tooth seats into a tempering furnace in time, and tempering at the high temperature of 750 ℃ for 2.5 hours.

(5) Rough and fine machining: and (3) carrying out internal spline broaching processing on the three-gear tooth seat forging stock after rough and finish turning of the outer circle and the end face to obtain a hot front finish turning semi-finished product.

(6) Carburizing and quenching treatment: and (3) loading the finished semi-finished product of the third gear tooth holder into a continuous carburizing furnace for carburizing treatment, discharging the semi-finished product at 830 ℃, and then placing the semi-finished product into a pressure quenching device for pressure quenching treatment, wherein the initial temperature of quenching oil is controlled to be 80 ℃.

(7) Low temperature tempering: and (3) placing the three-gear tooth seat after carburizing and pressure quenching in a tempering furnace, and performing low-temperature tempering treatment at 180 ℃ for 3 hours.

Compared with the existing isothermal normalizing process for producing the carburizing steel material and forging stock of the third gear tooth holder, the embodiment has the following beneficial effects:

(1) the free-cutting carburizing steel prepared by the embodiment has high Te and Bi yield, the sulfide length-width ratio of less than 3 can reach 80 percent, the raw material cost is low, and the cutting processability is good.

(2) The metallographic structure of the third gear tooth holder forging stock is ferrite and sorbite structure, the structure is fine and uniform, no abnormal structures such as bainite or martensite appear, the section hardness of the forging stock is controlled to be 176-179 HB, and the hardness dispersion difference is less than or equal to 3HB.

(3) The number of machined parts of the broach for broaching the internal spline of the three-gear tooth holder forging is increased from 800 to 1100, so that the production cost is reduced, and the production efficiency is improved.

(4) The qualification rate of the M value of the internal spline of the finished product of the third gear tooth holder is improved from 94.8% to 100%, so that the quality of the product is improved.

The foregoing is only a specific embodiment of the invention to enable those skilled in the art to understand or practice the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles herein may be applied to other embodiments without departing from the spirit or scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A bismuth tellurium adding method of free cutting carburizing steel is characterized in that,

the free-cutting carburizing steel comprises the following chemical components in percentage by weight: 0.16 to 0.22 percent of C, 0.10 to 0.30 percent of Si, 0.55 to 0.75 percent of Mn, 0.015 to 0.030 percent of P, 0.045 to 0.065 percent of S, 0.90 to 1.30 percent of Cr, 0.0018 to 0.0065 percent of Te, 0.015 to 0.035 percent of Ti, 0.010 to 0.015 percent of N, 0.009 to 0.039 percent of Bi, and the balance of Fe and unavoidable impurities, wherein the weight ratio of Te/S: 0.04 to 0.10 weight ratio of Bi/S: 0.25 to 0.60;

after adjusting other components except bismuth and tellurium at the end of molten steel refining, injecting a tellurium source and a bismuth source to the bottom of molten steel by compressing inert gas to prepare free-cutting molten steel containing bismuth and tellurium;

the injecting the tellurium source and the bismuth source to the bottom of the molten steel by compressing inert gas comprises: one end of a high-temperature resistant ceramic tube extends to the bottom of molten steel, and tellurium-manganese alloy particles and bismuth-manganese-iron alloy particles are sprayed to the bottom of molten steel through the high-temperature resistant ceramic tube by compressed inert gas;

the spraying of tellurium-manganese alloy particles and bismuth-manganese-iron alloy particles to the bottom of molten steel through a high-temperature resistant ceramic tube by using compressed inert gas comprises the following steps: firstly, rapidly and uniformly spraying tellurium-manganese alloy particles into molten steel in a VD furnace by using a high-temperature resistant ceramic tube, wherein the pressure of compressed inert gas is 30-50 kPa; after tellurium-manganese alloy particles are added, the temperature is kept for 40-50 s, then bismuth-manganese-iron alloy particles are quickly and uniformly sprayed into molten steel under the action of compressed inert gas by utilizing a high-temperature resistant ceramic tube, and the pressure of the compressed inert gas is 40-60 kPa; after bismuth ferromanganese is added into molten steel, the molten steel is kept for 150 to 200 seconds, so that alloy particles in the molten steel fully react and are melted;

the granularity of the tellurium-manganese alloy particles is 0.3-1.5 mm, the content of tellurium elements is 50% -60%, and the balance is manganese and unavoidable impurities; the granularity of the bismuth-manganese-iron alloy particles is 0.7-2.0 mm, the content of bismuth element is 60-70%, the content of manganese is 20-30%, the content of carbon is less than or equal to 2%, and the balance is Fe and unavoidable impurities.

2. The method for adding bismuth and tellurium to free-cutting carburizing steel according to claim 1, wherein the method comprises the steps of: the chemical components in percentage by weight are as follows: 0.17 to 0.21 percent of C, 0.19 to 0.27 percent of Si, 0.59 to 0.73 percent of Mn, 0.015 to 0.023 percent of P, 0.049 to 0.064 percent of S, 1.05 to 1.20 percent of Cr, 0.0039 to 0.0045 percent of Te, 0.022 to 0.028 percent of Ti, 0.011 to 0.012 percent of N, 0.018 to 0.037 percent of Bi0.018 and the balance of Fe and unavoidable impurities; wherein, te/S weight ratio: 0.06 to 0.09, bi/S weight ratio: 0.33 to 0.58.